In this study, we quantify the changes in terrestrial hydrology over the conterminous U.S. from 2005 to 2095 using the Community Land Model (CLM) driven by high-resolution downscaled climate and land use/land cover (LULC) scenarios. The regional climate is downscaled using a Regional Earth System Model (RESM) with boundary conditions provided by CESM historical simulation and future projections following the RCP 4.5 and 8.5 scenarios at 20km resolution, and then bias-corrected to 0.125 degrees (~12km) resolution. The LULC change projections from a sub-national version of the Global Change Assessment Model (GCAM) that resolves state-level details of the U.S. (GCAM-USA) are also downscaled to 0.05 degree resolution for the RCP scenarios and then aggregated to 0.125 degree resolution for input to CLM.
CLM is first run over 1975-2004 using the GCAM-USA downscaled LULC for year 2005 to simulate the hydrologic conditions in the historical period. For each RCP scenario, two CLM simulations are configured, one with the GCAM-USA downscaled LULC for the base year (i.e., 2005), the other with the time-varying GCAM-USA downscaled LULC projection from year 2005 to 2095 ingested at an annual time step, and both driven by the RESM downscaled climate. By analyzing key terrestrial hydrologic budget terms, such as runoff, evapotranspiration, root-zone soil moisture, and total terrestrial water storage, in response to simultaneous changes in climate and LULC versus climate alone, this study aims to provide insights to the relative impacts of climate and LULC changes on hydrologic processes and water resources in the U.S.